src: internalize per-isolate string properties

Speeds up property lookups a little and it creates the string in the
old space straight away.  It's a little easier on the garbage collector
because it doesn't have to track eternalized strings in the new space.

PR-URL: https://github.com/nodejs/node/pull/3060


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>